HTML Guide

The async and defer boolean attributes of the <script> element control how an external script should be executed once it has been downloaded. The async attribute makes sense when an external script (defined with the src attribute) is loaded, or when defining a script of type module:

<script async src="app.js"></script>

<script async type="module">
  /* JavaScript module code here */
</script>

The <script> tag allows authors to include dynamic scripts and data blocks in their documents. When the src is present, this tag accepts a type attribute which must be either:

• an empty string
• text/javascript (that’s the default, so it can be omitted)
• module

Examples:

<!-- This is valid, without a type it defaults to JavaScript -->
<script src="app.js"></script>

<!-- This is valid, but will warn that it can be omitted -->
<script type="text/javascript" src="app.js"></script>

<!-- An empty attribute is valid, but will warn that it can be omitted -->
<script type="" src="app.js"></script>

<!-- The module keyword is also valid as a type -->
<script type="module" src="app.js"></script>

<!-- Any other type is invalid -->
<script type="wrong" src="app.js"></script>
<script type="text/html" src="app.js"></script>
<script type="image/jpeg" src="app.js"></script>

The value contact is not a valid option for the autocomplete attribute on an <input> element.

The type dob is not valid for an input. If you want to build a date picker field, you can use the native HTML input elements with type date, datetime-local, or a generic text input decorated with JavaScript and CSS.

In HTML, the type attribute for the <input> element specifies the type of input control that is to be displayed. The type attribute can have values like text, password, email, date, etc. Using an unsupported or invalid value like dob (which presumably stands for “date of birth”) will cause this validation error.

Here’s an example of how you can correct this issue by using a supported type attribute value for the date of birth input:

<label for="dob">Date of Birth:</label>
<input type="date" id="dob" name="dob">

In this corrected example, we’ve used the type="date" attribute value for the date of birth input. This is a valid type for handling dates in HTML forms. Replace the input type with a supported type according to the specific data you need to capture.

Alternatively you can use a JavaScript library to build a date picker on a generic text input. For example, the popular bootstrap-datepicker library will generate a date picker around a text input.

All HTML elements may have the hidden boolean attribute set. When specified on an element, it indicates that the element is not yet, or is no longer, relevant, so browsers won’t render it.

Boolean attributes don’t accept values, its presence represents the true value and its absence represents the false value.

<!-- This is invalid because the hidden attribute should not have a value set -->
<div hidden="false"></div>

<!-- The correct way to hide a div is like this -->
<div hidden>This will be hidden</div>

<!-- And to show the element, we just don't hide it -->
<div>This won't be hidden</div>

An <a> element has been found with an invalid href attribute, containing more than one # adjacent character.

The # is used to separate the fragment part of an URI (typically used to indicate a section within a document). For example, this is a valid link to a URI containing a fragment:

<a href="https://example.com/faqs#pricing">pricing</a>

The next example is invalid because it contains two adjacent # characters, so that the fragment part would be #pricing instead of pricing:

<a href="https://example.com/faqs##pricing">pricing</a>

The href attribute of an <a> element contains an invalid character, that should be properly encoded as a URI percent-encoded character.

An illegal character has been found for the “href” attribute on the “link” element.

To fix this issue, find the “link” element in question and make sure that the “href” attribute contains a valid URL without any illegal characters.

Here’s some example HTML code of a link element:

<!DOCTYPE html>
<html>
  <head>
    <title>My Webpage</title>
    <link rel="stylesheet" href="styles/main.css">
  </head>
  <body>
    <h1>Welcome to my webpage!</h1>
    <p>Here is some content...</p>
  </body>
</html>

In the above example, the link element has a valid href attribute value of styles/main.css. Make sure that your href attribute values don’t contain any illegal characters.

For rel="preload" and as="image" only, the imagesizes attribute is a sizes attribute that indicates to preload the appropriate resource used by an img element with corresponding values for its srcset and sizes attributes.

For rel="preload" and as="image" only, the imagesrcset attribute is a sourceset attribute that indicates to preload the appropriate resource used by an img element with corresponding values for its srcset and sizes attributes.